Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. Patent Application No. 60/082,671, filed Apr. 22, 1998, and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation schemes. These pilot symbols are transmitted as quadrature phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames may propagate in a discontinuous transmission (DTX) mode. For voice traffic, transmission of user data occurs when the user speaks, but no data symbol transmission occurs when the user is silent. Similarly for packet data, the user data may be transmitted only when packets are ready to be sent. The frames are subdivided into sixteen equal time slots of 0.625 milliseconds each. Each time slot is further subdivided into equal symbol times. At a data rate of 32 KSPS, for example, each time slot includes twenty symbol times. Each frame includes pilot symbols as well as other control symbols such as transmit power control (TPC) symbols and rate information (RI) symbols. These control symbols include multiple bits otherwise known as chips to distinguish them from data bits. The chip transmission time (TC), therefore, is equal to the symbol time rate (T) divided by the number of chips in the symbol (N).
Previous studies have shown that multiple transmit antennas may improve reception by increasing transmit diversity for narrow band communication systems. In their paper New Detection Schemes for Transmit Diversity with no Channel Estimation, Tarokh et al. describe such a transmit diversity scheme for a TDMA system. The same concept is described in A Simple Transmitter Diversity Technique for Wireless Communications by Alamouti. Tarokh et al. and Alamouti, however, fail to teach such a transmit diversity scheme for a WCDMA communication system.
Other studies have investigated open loop transmit diversity schemes such as orthogonal transmit diversity (OTD) and time switched time diversity (TSTD) for WCDMA systems. Both OTD and TSTD systems have similar performance. Both use multiple transmit antennas to provide some diversity against fading, particularly at low Doppler rates and when there are insufficient paths for the rake receiver. Both OTD and TSTD systems, however, fail to exploit the extra path diversity that is possible for open loop systems. For example, the OTD encoder circuit of FIG. 5 receives symbols S1 and S2 on lead 500 and produces output signals on leads 504 and 506 for transmission by first and second antennas, respectively. These transmitted signals are received by a despreader input circuit (not shown). The despreader circuit sums received chip signals over a respective symbol time to produce first and second output signals Rj1 and Rj2 on leads 620 and 622 as in equations [1–2], respectively.                               R          j          1                =                                            ∑                              i                =                0                                            N                -                1                                      ⁢                                                  ⁢                                          r                j                            ⁡                              (                                  i                  +                                      τ                    j                                                  )                                              =                                                    α                j                1                            ⁢                              S                1                                      +                                          α                j                2                            ⁢                              S                2                                                                        [        1        ]                                          R          j          2                =                                            ∑                              i                =                N                                                              2                  ⁢                  N                                -                1                                      ⁢                                                  ⁢                                          r                j                            ⁡                              (                                  i                  +                                      τ                    j                                                  )                                              =                                                    α                j                1                            ⁢                              S                1                                      -                                          α                j                2                            ⁢                              S                2                                                                        [        2        ]            
The OTD phase correction circuit of FIG. 6 receives the output signals Rj1 and Rj2 corresponding to the jth of L multiple signal paths. The phase correction circuit produces soft outputs or signal estimates {tilde over (S)}1 and {tilde over (S)}2 for symbols S1 and S2 at leads 616 and 618 as shown in equations [3–4], respectively.                                           S            ~                    1                =                                            ∑                              j                =                1                            L                        ⁢                                                  ⁢                                          (                                                      R                    j                    1                                    +                                      R                    j                    2                                                  )                            ⁢                              α                j                                  1                  *                                                              =                                    ∑                              j                =                1                            L                        ⁢                          2              ⁢                                                                                      α                    j                    1                                                                    2                            ⁢                              S                1                                                                        [        3        ]                                                      S            ~                    2                =                                            ∑                              j                =                1                            L                        ⁢                                                  ⁢                                          (                                                      R                    j                    1                                    -                                      R                    j                    2                                                  )                            ⁢                              α                j                                  2                  *                                                              =                                    ∑                              j                =                1                            L                        ⁢                          2              ⁢                                                                                      α                    j                    2                                                                    2                            ⁢                              S                2                                                                        [        4        ]            Equations [3–4] show that the OTD method provides a single channel estimate a for each path j. A similar analysis for the TSTD system yields the same result. The OTD and TSTD methods, therefore, are limited to a path diversity of L. This path diversity limitation fails to exploit the extra path diversity that is possible for open loop systems as will be explained in detail.
Previous methods of diversity have also failed to exploit closed-loop power control between a mobile communication system and a remote base station. Present WCDMA power control for a single transmit antenna is best understood with reference to the signal flow diagram of FIG. 7 of the prior art. Sequential time slots 700–702 of the forward link signal from a base station to a mobile system include respective pilot symbols 704–706. These pilot symbols, for example pilot symbols 704, are transmitted at time tm to the mobile system. The mobile system receives the pilot symbols and produces a transmit power control (TPC) symbol. This TPC symbol is transmitted in the reverse link to the remote base station. The remote base station adjusts transmit power for the next forward link time slot 701 at time ts in response to this TPC symbol. Thus, the power control system of FIG. 7 fails to exploit advantages of closed-loop power control with path diversity.
By way of comparison, the signal flow diagram of FIG. 8 illustrates proposed power control for a TSTD system of the prior art. The TSTD system alternately transmits forward link time slots 800–802 from antennas A1 and A2. Pilot symbols 806 of time slot 800 are transmitted from antenna A1 at time tm1 followed by pilot symbols 807 of time slot 801 from antenna A2 at time tm2. Circuit 814 sums these pilot symbols and produces TPC symbol 816. This TPC symbol is transmitted in reverse link to remote the base station. The remote base station adjusts transmit power of antenna A1 at time ts of time slot 802 in response this TPC symbol. The TSTD method, however, is limited to a path diversity of L. Moreover, two time slots are required for each transmit power adjustment from time tm1 to time ts. Thus, the TSTD system has an additional disadvantage of imprecise power control due to increased time between received power measurement and transmit power adjustment.
Hosur et al. previously taught a new method for frame synchronization with space time transmit diversity (STTD) having a path diversity of 2L in U.S. patent application Ser. No. 09/195,942, filed Nov. 19, 1998, and incorporated herein by reference. Therein, Hosur et al. taught advantages of this increased diversity for WCDMA systems. Hosur et al. did not teach or suggest how this improved diversity might be used to improve closed-loop power control for WCDMA systems.